The present invention relates to a magnetoresistive head (MR head) used in a magnetic disk unit, etc., and more specifically, relates to a magnetoresistive sensor (MR sensor) for MR head, in which Barkhausen noise is effectively suppressed by exchange coupling of a magnetoresistive ferromagnetic layer (MR layer) with an antiferromagnetic layer, and a production method of the MR sensor.
Recently, the magnetic recording technique has made great progress. For example, it has been demanded to reduce the size and weight of a domestic VTR or VCR, and to reduce the size and highly increase the capacity of a magnetic disk unit. To meet the demand, a high recording density technique has now become being extensively studied. As the magnetic head for high density recording, an MR head having a magnetoresistive element as the sensing element is now putting into practical use.
The magnetic heads for high density recording brought into practice includes a ring magnetic head utilizing a ferrite single crystal in which the reading and writing are operated by a single head, a metal-in-gap magnetic head in which a magnetic layer of high saturation flux density is provided in a magnetic gap, an inductive thin film head made by utilizing photolithographic technique, and an MR reading/inductive writing head which consists of an inductive thin film head for writing and an MR head for reading. The study is being concentrated to the MR reading/inductive writing head because of its suitability for high density recording.
It would be effective for reducing the size of a magnetic disk unit to decrease the outer diameter of a magnetic recording medium such as a magnetic disk. However, this reduces the relative speed between the magnetic disk and the magnetic head, and as a result thereof, a sufficient output level becomes difficult to be obtained in the conventional electromagnetic inductive head such as a ring ferrite magnetic head and an inductive thin film magnetic head. On the other hand, since the MR head detects at least one of the amount and the direction of the signal field, the output level is constant without depending on the relative speed between the magnetic head and the magnetic disk. For this reason, the development of a highly efficient MR head is indispensable to meet the recently increasing demand for reducing the size of a magnetic disk unit.
FIG. 12 is a schematic illustration of a typical MR reading/inductive writing magnetic head. This illustration is a perspective view, from the side at which the magnetic recording medium is positioned, of the left half of the head taken along the center line thereof. An magnetoresistive element (MR element) 2 is disposed in a space between a bottom shield 1 on a substrate 20 and a middle shield 5 via two insulating layers (not shown), each in contact with the respective bottom shield 1 and middle shield 5, to form an MR reading head. On the upper side of the MR reading head, is mounted an inductive thin film writing head comprising a closed magnetic circuit constituted of a top writing magnetic pole 7 and the middle shield 5 and a writing coil 6. Both the heads are magnetically isolated from each other by the middle shield 5 so as to avoid the magnetic interference therebetween.
FIG. 13 is a schematic view of the side, opposite to a magnetic recording medium, of an MR sensor of a typical MR head. The MR element 2 has a multi-layered structure comprising a ferromagnetic MR layer 3 made of a magnetoresistive material such as NiFe-based alloy, etc., a space layer 9 made of a non-magnetic material having a relatively high electrical resistance such as Ta, a soft adjacent layer (SAL) 8 made of a soft magnetic material such as NiFeNb, NiFeCr, etc., and an antiferromagnetic layer 30, which is in direct contact with the MR layer 3, made of an NiMn-based alloy, etc. The SAL layer 8 provides the MR layer 3 with a transverse bias filed to maintain the reading operation linear. The antiferromagnetic layer 30 creates an antiferromagnetic exchange coupling with the MR layer 3 to suppress Barkhausen noise. An electrode layer 32 is connected to the antiferromagnetic layer 30. When a signal field as the external field is applied to the MR element, the electrical resistance of the MR layer 3 is reduced. As a result thereof, the voltage drop of the constant current from the electrode layer 32 becomes smaller and the change in the voltage drop is detected as the electrical signal corresponding to the signal field. The reference numerals 12, 21 and 23 show insulating layers and the reference numeral 18 shows a writing gap.
The problem usually encountered with the use of the MR head is reduction in S/N ratio due to Barkhausen noise. The sensor of the MR head which detects magnetic field has a film of a magnetoresistive ferromagnetic material, usually Permalloy (NiFe). A magnetic substance such as NiFe film, etc. is in a closure multi-domain state so as to minimize the magnetic energy. When a signal filed is applied thereto from a magnetic recording medium, the magnetic domain begins to rotate to cause the wall motion. When the wall motion is inhibited by the impurities or defects in the NiFe film, the magnetoresistive response curve becomes discontinuous and shows hysteresis. As a result, the wave form of output is distorted to cause noise. Thus, Barkhausen noise occurs.
To suppress Barkhausen noise, it is effective to induce a strong magnetic anisotropy in the lengthwise direction of the MR layer, thereby creating a single domain state in the MR layer. Hempstead et al. in IEEE Trans. Magn., MAG-14, 521 (1978) teach a method of aligning the spins in the NiFe film to the direction of the spins in the FeMn film. In this method, the spins of the NiFe ferromagnetic film are magnetically coupled with the spins of an antiferromagnetic .gamma.-FeMn film sputtered on the NiFe film by utilizing an interface exchange interaction. However, since the FeMn film is susceptible to corrosion and the blocking temperature (T.sub.B) at which the exchange coupling field (Hua) of the NiFe/FeMn films goes to zero is relatively low, i.e., 150.degree. C. or lower, the operating conditions and environments for the MR head must be strictly controlled.
Tsann Lin et al. in Appl. Phys. Lett., 65, (9), 1183 (1994) teach the use of an NiMn film in place of the FeMn film. It is taught therein that the T.sub.B is 400.degree. C. or higher, the Hua is 100 Oe or higher, and the corrosion resistance is superior to the FeMn film. Although the Hua of the as deposited NiFe/FeMn films is 20-30 Oe, the Hua of the as deposited NiFe/NiMn films is only several Oe, namely, the NiMn layer creates a very weak exchange coupling with the NiFe layer as compared with the FeMn layer. It is further reported that the heat treatment at a temperature higher than 240-250.degree. C. for at least several hours is required for increasing the Hua of the NiFe/NiMn films.
However, the inventors have found that the Hua of the NiFe/NiMn layers sufficiently increases in a certain case, but, not so increases in another case even when subjected to a sufficient heat treatment.